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Seismic Safety Regulations and Solutions

I have a special place in my heart for old buildings. Every college design course I took was related to new design. Concrete, steel, or wood design, the design problem was invariably part of a new building. I thought structural engineers designed new buildings. When I showed up for my first day of work wearing dress pants, a button-down shirt and a tie, I was handed a flashlight, tape measure, a clipboard and a Thomas Guide map (no Google maps back then) and sent to do as-built drawings for a concrete tilt-up that we were retrofitting.

When I was designing buildings, I created a lot of as-built drawings. Figuring out how a building was put together, what the structural system was (or wasn’t!) and designing a lateral load path in these old, and often historic buildings, was immensely satisfying. Knowing that history, it should not be surprising I have done a number of blog posts related to seismic retrofits. Soft-Story Retrofits, San Francisco’s Soft-Story Retrofit Ordinance, Remembering Loma Prieta, Resilient Communities, FEMA P-807, and Home Seismic Retrofit (there are probably a couple I forgot).

This week, Los Angeles Mayor Eric Garcetti proposed new seismic safety regulations . The recommendations are to retrofit soft-story wood-framed buildings within five years and older concrete buildings within 30 years. While these are only recommendations, it is encouraging to see politicians supporting policies to promote resiliency and life safety.

In San Francisco, thousands of building owners are already required by law to seismically retrofit multi-unit (at least five) soft-story, wood-frame residential structures that have two or more stories over a “soft” or “weak” story. These buildings typically have parking or commercial space on the ground floor with two or more stories above. As a result, the first floor has far more open areas of the wall than it actually has sheathed areas, making it particularly vulnerable to collapse in an earthquake.

Photo credit: J.K. Nakata and the U.S. Geological Survey

San Francisco’s ordinance affects buildings permitted for construction before Jan. 1, 1978. Mandatory seismic retrofit program notices requiring that buildings be screened were sent out in September, 2013, to more than 6,000 property owners. It is anticipated that approximately 4,000 of those buildings will be required to be retrofitted by 2020.

“When we look at the demographic of these buildings, they house approximately 110,000 San Franciscans. It’s paramount that we have housing for people after a disaster. We know we will see issues in all types of buildings, but this is an opportunity for us to be able to retrofit these buildings while keeping an estimated 1100,000 San Franciscans in their homes and, by the way of retrofit, allowing them to shelter in place after a disaster,” according to Patrick Otellini, San Francisco’s chief resilience officer and director of the city’s Earthquake Safety Implementation Program. “This exponentially kick starts the city’s recovery process.”

One solution to strengthen such buildings is the Simpson Strong-Tie® Strong Frame® special moment frame. Its patented Yield-Link™ structural fuses are designed to bear the brunt of lateral forces during an earthquake, isolating damage within the frame and keeping the structural integrity of the beams and columns intact.

“The structural fuses connect the beams to the columns. These fuses are designed to stretch and yield when the beam twists against the column, rather than the beam itself, and because of this the beams can be designed without bracing. This allows the Strong Frame to become a part of the wood building and perform in the way it’s supposed to,” said Steve Pryor, S.E., International Director of Building Systems at Simpson Strong-Tie. “It’s also the only commercially-available frame that bolts together and has the type of ductile capacity that can work inside of a wood-frame building.”

Installation of the Simpson Strong-Tie® Strong Frame® special moment frame

Another key advantage of the Simpson Strong-Tie special moment frame is no field welding is required, which eliminates the risk of fire in San Francisco’s older wood-framed buildings.

To learn more about San Francisco’s retrofit ordinance, watch a new video posted on strongtie.com/softstory. For more information about the Strong Frame special moment frame, visit strongtie.com/strongframe.

City of San Francisco Implements Soft-Story Retrofit Ordinance

The city of San Francisco is a unique construction environment that is 98% built-out with little new residential construction. Protecting the existing structures is particularly imperative to save the character of the city and maintain the population base by preventing a major migration out of the city after a large seismic event.

The Community Action Plan for Seismic Safety (CAPSS) was a San Francisco Department of Building Inspection 10-year-long study started in 1998 to study earthquake risks in San Francisco and develop suggestions for mitigating loss of life and property damage from future earthquakes. I once worked as a volunteer on a sidewalk survey to inventory soft-story buildings as part of CAPSS. The key recommendations of CAPSS evolved into the CAPSS Earthquake Safety Implementation Program (ESIP), which has 50 tasks that will be implemented over the next 30 years.

One of those tasks includes the Mandatory Soft-Story Retrofit Program, which Mayor Ed Lee signed into law on April 18, 2013 at the annual commemoration of the 1906 San Francisco Earthquake. The law requires wood-framed buildings that:

Were built prior to 1978
Are three stories or more and,
Have five or more dwelling units

to be evaluated and brought into compliance. The city sent notices to building owners on September 15. It’s estimated that there are between 4,000 and 10,000 structures that fall under the program.

Unreinforced Masonry (URM) Buildings: Seismic Retrofit

Unreinforced masonry (URM) buildings in moderate- to high-seismic areas can be a disaster in waiting. These types of structures have very little of the ductility required of structures to prevent loss of life or business disruption in a seismic event. (Consult our Structural Engineering Blog post “Building Drift – Do You Check It?” for a discussion on ductility.) Many of these buildings are in densely populated areas, have historical meaning, provide important living or business spaces, and can be costly to retrofit. In this blog, Simpson Strong-Tie engineers discuss tools available for engineers to assess these buildings and design the retrofits needed to mitigate a potential loss of life and increase seismic resiliency.

Soft-Story Retrofits

In February 2007 I had the opportunity to volunteer for a Soft-Story Sidewalk Survey for the San Francisco Department of Building Inspection. The purpose of the survey was to inventory buildings in San Francisco that appeared superficially to have soft or weak first stories. The volunteers were given a list of addresses to review and we recorded if the building was more than three stories tall, had five or more dwellings, and estimated what percentage of the ground level had openings in the walls. No structural analysis going on, just counting stories, mailboxes, doors and windows.

San Francisco soft-story structure. Photo credit: USGS. — San Francisco soft-story structure failure. Photo credit: USGS.

A collapsed house in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images. — A collapsed soft-story in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.

“Sunny With A Chance Of Earthquake”

With scientists predicting a 99% chance of a 6.7 magnitude earthquake striking Southern California within the next 30 years, California weather forecasters may want to remind residents that the location which gives lots of golden sunshine year round is also one of high seismic risk.

Recent earthquakes in cities similar to Los Angeles, San Francisco and Memphis have served as a reminder of the damage and disruption a major earthquake can have on a community. Through lessons learned from past earthquakes and research performed by the construction industry, there are many tools available geared toward residential buildings to reduce damage and resist collapse.